1. Field of the Invention
The invention relates to a method and apparatus for inspecting an object to be inspected. For example, the invention is suitable for an inspection method and a surface inspection apparatus for detecting a foreign matter, a defect, and the like in an object to be inspected such as a semiconductor wafer, a glass substrate or a ceramic substrate.
2. Description of the Related Art
In the case where a foreign matter or a defect exists in the surface of a semiconductor wafer, it exerts an influence on the yield of a semiconductor device. Consequently, a semiconductor wafer surface inspection has been being performed by a surface inspection apparatus. As conventional techniques for detecting a foreign matter and a defect existing in the surface of a wafer, techniques disclosed in U.S. Pat. No. 6,201,601 and Japanese Patent Application Laid-Open Publication No. 11-153549 (JP-A-11-153549) are known.
U.S. Pat. No. 6,201,601 discloses a surface inspection apparatus for irradiating a wafer with a perpendicular beam and an oblique beam from an illumination optical system using a laser as a light source, collecting scattered light from the wafer by a parabolic mirror, and detecting the collected light by a detector. Scattered light originating from the perpendicular beam and scattered light originating from the oblique beam is split from each other by intentionally introducing an offset between the two radiation beams, using two beams having different wavelengths, or switching the perpendicular radiation beam and the oblique radiation beam on and off alternately. A beam position error caused by a change in sample height is corrected by detecting specular reflection of the oblique radiation beam and changing the radiation direction in accordance with a result of detection of the specular reflection by a mirror.
JP-A-11-153549 discloses a method of inspecting the surface of an object to be measured, by emitting light from a light source via an optical system obliquely to the surface of an object to be measured, receiving scattered light reflected from the surface of the object to be measured, while making the object to be measured and the optical system displaced relative to each other, detecting a foreign matter on the surface of the object to be measured, and recording the coordinate position of the foreign matter. In the method, at the time of detecting a foreign matter on the surface of the object to be measured, height of the object to be measured is measured. By using a height signal of the object to be measured, the coordinate position of the foreign matter is corrected.
A control criterion for a foreign matter and a defect existing in a semiconductor wafer is becoming severer as the size of a semiconductor device is becoming smaller. In recent years, since even a foreign matter adhered to the rear face of a wafer and a rear-face state exert an influence on the yield of a semiconductor device, an inspection for a foreign matter and a defect existing not only in the main face of a wafer but also in the rear face is demanded. In such an inspection, a stage of an edge grip method of handling the main face and rear face of a wafer in a non-contact manner is used. However, since an internal space which makes the main face and rear face in non-contact state is provided and a wafer itself is held by its edges, a large deformation (deflection and warp) occurs in the wafer due to pressure fluctuations in the internal space accompanying self weight and rotation of the wafer. The deformation of the wafer makes both sensitivity of detection of a foreign matter and a defect and the coordinate precision significantly degrade. It is therefore necessary to detect a deformation state under operation and to correct the state in order to maintain the detection sensitivity and coordinate precision.
However, in the conventional techniques, detection of a wafer deformation state while detecting a foreign matter and a defect existing in the wafer is not considered. There is a problem such that it is not possible to determine whether or not the shape of a wafer is in a proper range in order to determine credibility of detection sensitivity and position coordinate precision of a surface inspection apparatus and in order to maintain the performance of detection sensitivity and position coordinate precision. In addition, since correction of the shape of a wafer against the detected deformation state of a wafer is not considered, deformation-following ability of an autofocus mechanism degrades. Due to a focus deviation and an irradiation position deviation of a perpendicular irradiation beam spot and an oblique irradiation beam spot formed on the surface of a wafer, detection sensitivity of a foreign matter and a defect and position coordinate precision degrade. When deformation of a wafer under inspection is conspicuous, a problem occurs such that a small foreign matter and a small defect cannot be detected.